Implantable medical devices are available for treating cardiac arrhythmias by delivering electrical stimulation therapy for pacing, cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation or to detect a need for bradycardia pacing.
Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to a pathologically low rate or absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. A pacing pulse must be of adequate energy to depolarize or “capture” the cardiac tissue. The lowest pacing pulse energy which captures the heart is referred to as the pacing or capture threshold.
In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
The performance of an ICD depends on accurate sensing of intrinsic cardiac activity, such as P-waves and R-waves, in order to reliably detect arrhythmias. Typically, cardiac electrogram (EGM) sensing is performed in a bipolar manner between a “tip” electrode located at the distal end of a cardiac lead and a “ring” electrode spaced proximally from the tip electrode, or in a unipolar manner between either a tip or ring electrode and the ICD housing, referred to as a “can” or “case” electrode. Pacing is generally delivered using the tip electrode paired with either the ring electrode or ICD housing. Integrated bipolar leads are also known for use with ICDs in which a coil electrode is used, in place of the ring electrode, in combination with the tip electrode for bipolar pacing or sensing.
One limitation encountered when sensing cardiac signals using the same electrodes that are used for pacing is related to the afterpotential signal following delivery of a pacing pulse. Polarization at the electrode-tissue interface causes an afterpotential signal that can saturate sense amplifiers included in the cardiac pacing device and mask an evoked response signal. Sense amplifier circuitry may be temporarily disabled during and immediately following a pacing pulse to avoid saturation of sense amplifiers and the erroneous detection of the pacing pulse and post-pulse polarization artifacts. Reference is made, for example, to U.S. Pat. No. 4,379,459 issued to Stein. The post-pulse polarization artifact diminishes during the blanking interval, however, during this time the device is “blinded” to sensing cardiac signals, making detection of intrinsic or pacing-evoked depolarizations difficult.
In commercially available systems, capture is typically verified by sensing the evoked response following a pacing pulse. The challenges of sensing evoked cardiac signals are well-known in the art and have been addressed by a number of approaches in order to provide reliable capture management. Sensing the evoked response using an electrode pair that is different than the electrode pair used for delivering the pacing pulse can reduce or eliminate polarization artifact interference. Sensing a far-field signal related to an evoked response, as opposed to the near-field evoked response signal, or sensing a conducted depolarization away from the pacing site have also been proposed. See for example, U.S. Pat. No. 5,324,310 issued to Greeninger, U.S. Pat. No. 5,222,493 issued to Sholder, U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 6,434,428 issued to Sloman, et al., and U.S. Pat. App. No. 20010049543, issued to Kroll. Depending on the associated lead system in use and the presence of conduction abnormalities, however, sensing far-field or conducted depolarizations using alternative sensing electrodes may not always be possible.
Regardless of the approach used for sensing pacing-evoked depolarizations, sense amplifier blanking during and after a pacing pulse blinds the device to sensing intrinsic depolarizations. At relatively high pacing rates, the duration of sense amplifier blanking during and after a pacing pulse may become substantial compared to the duration that the sense amplifier is enabled, significantly reducing the amount of time that the device is able to detect intrinsic activity. High rate intrinsic activity, such as during tachycardia or fibrillation, may consequently go undetected. Therefore, the maximum allowable pacing rate is typically limited in commercially available ICDs such that the detection of high rate arrhythmias is not impaired.
Rate-responsive pacemakers provide automatic adjustments to the pacing rate in response to a sensed signal indicative of the metabolic need of the patient. Rate-response sensors known for use with implantable cardiac stimulation devices include, for example, piezoelectric activity sensors and thoracic impedance sensing for estimating minute volume. Reference is made to U.S. Pat. No. 4,485,813 issued to Anderson, et al., and U.S. Pat. No. 4,702,253 issued to Nappholz et al. As the pacing rate is increased in response to increased metabolic need, the time in which the device is able to sense intrinsic activity is reduced when the sense amplifier blanking interval is fixed. Therefore, the device may limit the maximum rate-responsive pacing rate in order to preclude impaired sensing of intrinsic activity and arrhythmia detection. For example, the maximum upper pacing rate may correspond to a pacing escape interval equal to twice the sense amplifier blanking period, such that sensing is disabled no more than 50% of the time. Pacing at rates within tachycardia or fibrillation detection zones is typically avoided.
In some situations, however, this pacing rate constraint may prevent the physiological needs of the patient from being met, particularly in young, active patients. Relatively high pacing rates may be required to meet the metabolic needs of the patient, but higher rate pacing may be unavailable because of the pacing rate limitations imposed in order to prevent undersensing of high-rate intrinsic cardiac activity. Thus, it is desirable to provide rate-responsive pacing in ICDs without over-restricting the maximum pacing rate limit while still providing reliable detection of arrhythmias, and that reliably sense intrinsic cardiac activity during high rate cardiac stimulation.